drm: fb_helper: improve CONFIG_FB dependency

My previous patch correctly addressed the possible link failure, but as
Jani points out, the dependency is now stricter than it needs to be.

Change it again, to allow DRM_FBDEV_EMULATION to be used when
DRM_KMS_HELPER and FB are both loadable modules and DRM is linked into
the kernel.

As a side-effect, the option is now only visible when at least one DRM
driver makes use of DRM_KMS_HELPER. This is better, because the option
has no effect otherwise.

Fixes: 606b102876 ("drm: fb_helper: fix CONFIG_FB dependency")
Suggested-by: Acked-by: Jani Nikula <[email protected]>
Reviewed-by: Javier Martinez Canillas <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

drivers/gpu/drm/Kconfig

@@ -117,9 +117,8 @@ config DRM_DEBUG_MODESET_LOCK
 
 config DRM_FBDEV_EMULATION
 	bool "Enable legacy fbdev support for your modesetting driver"
-	depends on DRM
-	depends on FB=y || FB=DRM
-	select DRM_KMS_HELPER
+	depends on DRM_KMS_HELPER
+	depends on FB=y || FB=DRM_KMS_HELPER
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT